Some containers for containing liquid contents such as drinks are made of various materials. A synthetic resin bottle, for example, a bottle blow-molded of polyethylene terephthalate (or a PET bottle), is known as a type of container as described above. Patent Literature 1 discloses a method for forming a PET bottle from a preform as a prototype by a blow molding process. Specifically, a closed-end cylindrical preform having an opening at one end and closed at the other end is heated, and a body portion of the preform is inserted into a mold formed in the same shape as an outer shape of the PET bottle, with a neck portion of the preform on the opening side thereof gripped. Then, high-pressure air is introduced into the preform through its opening to expand the preform so as to bring the preform into intimate contact with the inside of the mold, thereby forming the PET bottle. The PET bottle, when undergoing quality inspection, is divided into plural cutoff pieces by being cut in section for each of inspection parts.
FIG. 7 is a view of a state of a PET bottle as cut for each of inspection parts. In an example of FIG. 7, a PET bottle 10 is divided into four cutoff pieces 10a to 10d. The quality inspection involves measuring the cutoff pieces 10a to 10d for weight, wall thickness, buckling strength or the like, and comparing measured results with a preset set value, thereby inspecting the PET bottle for quality. When there is a great difference between the measured results and the set value, the results of the quality inspection are fed back into the blow molding process to adjust a condition for blow molding, such as the temperature of the preform. This improves the yield rate of the PET bottle. For example, a part in which the wall thickness is less than the set value can be considered to be a thin-walled part of the preform as compared to other parts. This can be considered to be due to the fact that the thin-walled part of the preform is prone to conduct heat and thus, in the blow molding process, the thin-walled part rises high in its temperature relatively to other parts and hence expands larger than other parts by the introduction of the high-pressure air. Therefore, setting the temperature of the thin-walled part low enables the wall thickness to approach the set value.